Forward Range

`X`	A type that is a model of Forward Range.
`a`	Object of type X.

A range X where boost::range_iterator<X>::type is a model of Forward Traversal Iterator.

Distance type	`boost::range_difference<X>::type`	A signed integral type used to represent the distance between two of the Range's iterators. This type must be the same as the iterator's distance type.
Size type	`boost::range_size<X>::type`	An unsigned integral type that can represent any nonnegative value of the Range's distance type.